1. Field of the Invention
The present invention provides an improved eddy current probe that is particularly useful for inspecting the interior of a combustion turbine without disassembly of the turbine.
2. Description of the Related Art
Eddy current testing of electrically conductive material is typically performed by exposing the material to an alternating magnetic field generated by a coil carrying an alternating current. The magnetic field generates small electric currents at or near the surface of the material, commonly known as eddy currents. The eddy currents generate their own magnetic fields, which, when they interact with the magnetic field of the exciting coil, will change the electrical impedance of the coil. The impedance of the exciting coil may be measured, and is compared with the impedance of a separate indicating coil, thereby detecting any condition that would affect the conductivity of the test material. Such conditions include cracks, voids, inclusions, seams, and stress concentrations at or near the surface of the material. Additionally, differences in metal chemistry and/or heat treatment of the metal will affect the conductivity and magnetic permeability of the test material, and may therefore be detected by eddy current inspection. The thickness of platings, coatings, and/or corrosion also lend themselves to eddy current measurements.
An eddy current inspection system includes a coil contained within a probe, for carrying alternating currents and inducing eddy currents in the part being tested; a coil for sensing the magnetic field changes caused by the interaction of the eddy currents with the original magnetic field, which may be either the exciting coil or a separate sensing coil; and a means of measuring and interpreting the resulting impedance changes, for example, measuring the induced voltage of the sending coil. Different types of eddy current probes include absolute coils, which consist of a single winding where the impedance or induced voltage in the coil is measured directly; a differential coil, which utilizes a pair of opposing coils, and compares the change in inductance within one coil to the change in inductance within the other coil; and torroid coils, which directly monitor the driving coil and measures changes in the impedance of the coil due to changes in conductivity and permeability.
The inaccessibility and small size of the open space within the combustion turbine, and between and among the turbine blades, makes inspection of the blades difficult without partial disassembly of the turbine. Presently available eddy current probes, configured for insertion through the inspection ports on the turbine, are unable to reach all portions of a blade within the turbine that need to be inspected. Additionally, it is difficult to determine the exact location of such probes within the turbine, thereby creating a possibility that certain regions that need to be inspected will be missed during the inspection.
Accordingly, there is a need for an eddy current probe that is adapted to reach all areas of a blade within a turbine through an inspection port. Additionally, there is a need for an eddy current inspection probe tip configured for inspecting different portions of the blade having different geometric shapes. Furthermore, there is a need for an eddy current probe that facilitates precise determination of its location within the turbine.